1. Field of the Invention
The present invention relates to an image forming apparatus applied to a printing unit of a digital copying machine, a facsimile or the like as well as to a digital printer, a plotter or the like, for forming an image on a recording medium by jetting a developer.
2. Description of the Background Art
Recently, an image forming apparatus has been proposed which outputs image signals as a visible image on a recording medium such as a sheet of paper, as described, for example, in Japanese Patent Laying-Open No. 4-286662. In the apparatus, an electric field is exerted on charged particles so that the particles are jet out by electric force, and the charged particles reach and adhere to the recording medium, as a potential applied to a control electrode including a plurality of through holes arranged in the jet travel path is changed. In this manner, an image is directly formed on the recording medium. In addition, the control electrode is driven by a driver, and a developer removing electric field is formed in the through holes of the control electrode, whereby the developer left in the through holes of the control electrode is removed.
In the conventional image forming apparatus having the above described structure, means for controlling passage of charged particles through a gate is used. In such an image forming apparatus, whether charged particles (toner) are allowed to jet out or not is controlled by controlling an electric field generated between the gate and a tone carrier, and, by a strong electric field generated by an opposing electrode, the toner is attracted to reach the surface of the sheet, which is the recording medium.
In the above described image forming apparatus, the toner inevitably adheres on the control electrode. The toner adheres not only to the surface of the control electrode facing the carrier but also the inside of the gate through which toner passes and to that side of the control electrode which faces the opposing electrode. Adhesion of toner causes various problems.
For example, the potential to be applied to the control electrode is shielded by the toner. In addition, the potential of the control electrode varies because of the potential caused by the charges held by the toner. More specifically, the potential is changed to a state which does not allow jetting of toner, resulting in unsuccessful toner jetting. As jetting of the toner is not satisfactory, resulting dots may have their diameters varied, and density inappropriate, readily resulting in degraded image quality.
Further, in such a state, a desired electric field cannot be generated around the gate, and therefore the course of travel of the jetting toner considerably deviates from a satisfactory course, and therefore it becomes difficult to control the course of travel of the jetting toner. Under such condition, it is difficult to guide the toner to a prescribed position. As a result, scattering of the toner is apt to occur, degrading quality of resulting dots, and, as already mentioned above, the diameters and densities of the dots may vary.
In such a situation, image formation of desired halftone is difficult, and satisfactory color reproduction is difficult in a color image forming apparatus. Further, if adhesion of the toner to the control electrode takes place in the gate which is the passage for the toner, it hinders passage of the toner through the gate, so that a prescribed amount of toner cannot pass through the gate, resulting in the above described problems. Further, the course of travel of the jetting toner changes by contact between the passing toner and the toner adhered to the inside of the gate, or by an electric field generated by the charges held by the toner adhered inside the gate. Therefore, satisfactory control of the course of travel of the jetting toner becomes difficult as in the above, resulting in similar problems. In addition, excessive adhesion of the toner to the control electrode eventually causes clogging of the gate. This physically prevents passage of the toner, readily resulting in unsatisfactory image formation or blank.
If a portion of the sheet should contact the control electrode because of a wrinkle or curl of the sheet, the toner adhered on the control electrode on the opposing electrode side touches and stains the sheet. Further, it may tarnish the formed image.
Now that adhesion of the toner to the control electrode is unavoidable and the adhered toner causes various problems, it is necessary to clean the control electrode of the adhered toner. In the conventional image forming apparatus as represented by the one described above, an AC potential is applied to the control electrode to form a cleaning electric field, which is an AC electric field having a DC component between the control electrode and the opposing electrode or the carrier to remove the toner.
However, the toner contains not only the toner of a desired charge polarity but also toner having the charge polarity opposite to the desired polarity. It is possible by the method of cleaning toner represented by the above described prior art to remove the toner having a prescribed charge polarity (hereinafter referred to as positively charged toner). It is difficult, however, to remove the toner having charge polarity different from the positively charged toner (hereinafter referred to as negatively charged toner).
In order to remove the negatively charged toner, it is necessary to reverse the polarity of the DC component in the cleaning electric field, and, in that case, it becomes difficult to clean the positively charged toner. Accordingly, what is possible by the cleaning electric field is only to remove the toner of one polarity, and simultaneous cleaning of the toners having opposite polarities has been difficult. Therefore, toners of respective charge polarities have to be cleaned separately. If cleaning is done in an interval of paper feeding, the time required is twice as long as that necessary for cleaning toner having single polarity. This time requirement makes it difficult to shorten the interval between paper feeding, and hence hinders improvement in speed of printing.
The present invention was made to solve the above described problems, and its object is to provide an image forming apparatus capable of high speed printing.
Another object of the present invention is to provide an image forming apparatus capable of high speed printing ensuring highly effective cleaning and free of image degradation.
Another object of the present invention is to provide an image forming apparatus capable of high speed printing, reduction in number of parts, reduction in size and cost, and improvement in reliability.
The image forming apparatus in accordance with an aspect of the present invention includes a toner carrier for carrying toner, an opposing electrode arranged opposing to the toner carrier, a high voltage power source unit for supplying a voltage to generate potential difference between the toner carrier and the opposing electrode, a control electrode including a plurality of electrodes arranged between the toner carrier and the opposing electrode, and a control power source unit for implementing a plurality of potential states of respective electrodes of the control electrode, the control electrode including an insulating substrate, the aforementioned plurality of electrodes each having a passage portion for the toner provided on the insulating substrate, and a dielectric layer formed on the side of the opposing electrode such that attracting force Fc exerted on the toner adhered on that surface of the control electrode which faces the opposing electrode is made smaller than attracting force Fb exerted on the toner adhered on the opposing electrode, and the control power source unit including an image forming unit for applying prescribed potentials to respective ones of the plurality of electrodes to control passage of the toner through the passage portion for forming an image on a surface of a recording medium conveyed over the opposing electrode, and an electric field applying unit for applying an electric field of which electric field direction changes between the opposing electrode and the control electrode and which ensures that electric force FE exerted on the toner adhered on the control electrode is greater than the attracting force Fc and that the electric force FE is smaller than the attracting force Fb.
When the toner reciprocates between the opposing electrode and the control electrode, the toner on the control electrode moves to the surface of the opposing electrode and eventually the control electrode is clean of the toner, as the toner moves to the opposing electrode because of difference in attracting force. In such a structure, cleaning of toner is possible regardless of the polarity of the toner. Therefore, there is not at all the influence of the negatively charged toner, and the control electrode can be cleaned by a single cleaning operation. Accordingly, it becomes unnecessary to repeat the cleaning operation for toners of different polarities to be cleaned. This means that the interval between paper feeding can be reduced and that the speed of printing is improved.
Preferably, the electric field applying unit includes a rectangular wave potential applying circuit for applying a rectangular wave potential to generate an electric field which ensures that the electric force FE received by the toner adhered on the control electrode is higher than the attracting force Fc and that the electric force FE is lower than the attracting force Fb.
When the rectangular wave potential is applied, the direction of the electric field changes in a moment, causing a significant amount of change in the electric field. This provides the same effect as caused by oscillation over extremely wide frequency range, further enhancing the effect of cleaning. As the effect of cleaning is enhanced, degradation in image quality is better suppressed.
More preferably, the rectangular wave potential applying circuit includes a circuit for applying the rectangular wave potential to generate an electric field not having any DC component which ensures that the electric force FE received by the toner adhered on the control electrode is greater than the attracting force Fc and that the electric force FE is smaller than the attracting force Fb.
Since the electric field does not have any DC component, both the positively charged toner and negatively charged toner can equally be removed, enabling effective cleaning. Accordingly, degradation of image quality is prevented.
More preferably, the toner, the toner carrier, the opposing electrode and the control electrode include a plurality of different toners, a plurality of toner carriers, a plurality of opposing electrodes and a plurality of control electrodes, respectively. The electric field applying unit includes circuits for applying a plurality of electric fields in accordance with respective characteristics of the plurality of different toners, the direction of which electric fields change respectively between the plurality of opposing electrodes and the plurality of control electrodes and which ensure that a plurality of electric forces FFE received by the plurality of different toners adhered on the plurality of control electrodes are greater than the attracting forces FFc of the plurality of different toners adhered on that surface of the plurality of control electrodes facing the opposing electrodes, and that the plurality of electric forces FFE are smaller than the attracting forces FFb of the plurality of different toners adhered on the plurality of opposing electrodes, respectively.
The electric field is changed in accordance with the characteristic of the toner used. This ensures effective cleaning and prevents degradation of image quality.
More preferably, the toner, the toner carrier, the opposing electrode and the control electrode include a plurality of different toners, a plurality of toner carriers, a plurality of opposing electrodes and a plurality of control electrodes, respectively. The electric field applying unit includes a circuit for applying a single electric field satisfying a prescribed condition, the dielectric layer of each of the plurality of control electrodes includes a dielectric layer having a dielectric constant or thickness adjusted to satisfy the prescribed condition when the single electric field is applied by the electric field applying unit, each of the plurality of different toners includes a toner having its charge amount adjusted to satisfy the prescribed condition when the single electric field is applied by the electric field applying unit, and the prescribed condition is as follows. The direction of the electric field changes between the plurality of opposing electrodes and the plurality of control electrodes, respectively, a plurality of electric forces FFE received by the plurality of different toners adhered on plurality of control electrodes respectively are greater than attracting forces FFc of the plurality of different toners adhered on that surface of the plurality of control electrodes that faces the opposing electrodes, and that the plurality of electric forces FFE are smaller than attracting forces FFb of the plurality of different toners adhered on the plurality of opposing electrodes.
The characteristic of the control electrode or the toner is adjusted to enable cleaning of all control electrodes by a single electric field. Accordingly, it becomes unnecessary to provide a plurality of power sources for cleaning respective control electrodes. This enables reduction in the number of parts, size, cost and improves reliability.
More preferably, the image forming apparatus further includes a cleaning unit for removing the toner adhered on the surface of the opposing electrode.
It is possible to clean the toner which has moved from the control electrode to the opposing electrode. Therefore, an image forming apparatus is provided in which staining of a rear surface of the sheet by the toner adhered on the surface of the opposing electrode is prevented.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.